In many internal combustion engines the engine cylinder intake and exhaust valves may be opened and closed by fixed profile cams in the engine, and more specifically by one or more fixed lobes which may be an integral part of each of the cams. The use of fixed profile cams makes it difficult to adjust the timings and/or amounts of engine valve lift to optimize valve opening times and lift for various engine operating conditions, such as different engine speeds. Sophisticated engine control, however, requires variable valve timing and variable valve lift. Furthermore, valve opening and closing velocity should be controlled.
One method of adjusting valve timing and lift, given a fixed cam profile, has been to incorporate a "lost motion" device in the valve train linkage between the valve and the cam. Lost motion is the term applied to a class of technical solutions for modifying the valve motion proscribed by a cam profile with a variable length mechanical, hydraulic, or other linkage means. In a lost motion system, a cam lobe may provide the "maximum" (longest dwell and greatest lift) motion needed over a full range of engine operating conditions. A variable length system may then be included in the valve train linkage, intermediate of the valve to be opened and the cam providing the maximum motion, to subtract or lose part or all of the motion imparted by the cam to the valve.
This variable length system (or lost motion system) may, when expanded filly, transmit all of the cam motion to the valve, and when contracted fully, transmit none or a minimum amount of the cam motion to the valve. Examples of such a system and method are provided in Vorih U.S. Pat. No. 5,829,397 and Hu U.S. Pat. No. 5,537,976, which are assigned to the same assignee as the present application, and which are incorporated herein by reference.
In a lost motion system an engine cam shaft may actuate a master piston which displaces fluid from its hydraulic chamber into a hydraulic chamber of a slave piston. The slave piston in turn acts on the engine valve to open it. The lost motion system may be a solenoid valve and a check valve in communication with the hydraulic circuit including the chambers of the master and slave pistons. The solenoid valve may be maintained in a closed position in order to retain hydraulic fluid in the circuit. As long as the solenoid valve remains closed, the slave piston and the engine valve respond directly to the motion of the master piston, which in turn displaces hydraulic fluid in direct response to the motion of a cam. When the solenoid is opened temporarily, the circuit may partially drain, and part or all of the hydraulic pressure generated by the master piston may be absorbed by the circuit rather than be applied to displace the slave piston.
Prior to the present invention, few lost motion systems have provided fully variable degrees of valve lift and dwell. Such variability in lost motion systems has been attained by rapid release of hydraulic pressure from the slave piston in order to close the engine valve connected to the slave piston. Valve closing motions that are dictated by the rapid release of hydraulic pressure tend to result in undesirably high valve closing velocities. This results in unacceptably short closing durations at low speed. There is a need for a lost motion system in which valve closing angles may be kept constant through the engine speed range. This device may be used with valve seating control devices to control valve seating velocities.
Previous lost motion systems have used a single cam to drive the master piston--slave piston combination in the system. Accordingly, valve motion must either come from a direct hydraulic following of the single cam profile, or some version of that profile minus the motion "lost" by the system. Controlled loss of hydraulic actuation may require complicated control valves and controllers capable of throttling the release of hydraulic pressure from the salve piston. Controlled loss of hydraulic actuation may also require selection of a system tuned to provide optimum release of hydraulic pressure for only one set of engine conditions.
In the present invention, high speed control valves may switch the cam profile that is hydraulically connected to the slave piston. Thus, the system may provide a range of valve actuation from multiple cam profiles. By using high speed mechanisms to select particular cam profiles for valve opening and closing, more precise control may be attained over valve actuation, and accordingly optimal valve actuation may be attained for a wide range of engine operating conditions.
Applicant has determined that the lost motion system and method of the present invention may be particularly useful in engines requiring valve actuation for both positive power and for compression release retarding and exhaust gas recirculation valve events.
An example of a lost motion system and method used to obtain retarding and exhaust gas recirculation is provided by the Gobert, U.S. Pat. No. 5,146,890 (Sep. 15, 1992) for a Method And A Device For Engine Braking A Four Stroke Internal Combustion Engine, assigned to AB Volvo, and incorporated herein by reference. Gobert discloses a method of conducting exhaust gas recirculation by placing the cylinder in communication with the exhaust system during the first part of the compression stroke and optionally also during the latter part of the inlet stroke. Gobert uses a lost motion system to enable and disable retarding and exhaust gas recirculation, but such system is not variable within an engine cycle.
None of the lost motion systems or methods of the prior art have enabled precise control of valve actuation to optimize valve movement for different engine operating conditions. Furthermore, none of the lost motion systems or methods of the prior art disclose, teach or suggest the use of high speed control valves to switch the cam profile driving a slave piston during a single valve event. Independent control over valve lift and dwell may be realized by cam profile switching. In addition, none of the prior art discloses, teaches or suggests any system or method for using such a cam profile switching arrangement to control and/or reduce engine valve seating velocities.
Accordingly, there is a significant need for a system and method of controlling lost motion which: (i) optimizes engine operation under various engine operating conditions; (ii) provides precise control of lost motion; (iii) provides acceptable valve closing velocities; and (iv) is capable of providing all intake and exhaust valve events.